1. Field of the Invention
The subject invention relates to circuitry for rank ordering a set of numbers, and particularly to such a circuit having the capability to adjust rank in a single clock cycle. The circuit features improved flexibility over the prior art and is particularly applicable to radars which employ OS CFAR detection, while having broad applications in many other number ranking contexts.
2. Description of Related Art
An improved method of computing thresholds for a radar detector is described in "Radar CFAR Thresholding in Clutter and Multiple Target Situations" by Herman Rohling in IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-19, No. 3, July 1983. This method is referred to as OS CFAR, which stands for "ordered statistic constant false alarm rate." This OS CFAR method offers several important advantages over presently-used CFAR (Constant False Alarm Rate) methods. Such presently-used methods employ a CFAR circuit which computes a mean level based on the arithmetic average of the video in the area around the sample of interest. As explained by Rohling, this cell averaging method does not perform well in the presence of clutter or multiple targets. An Ordered Statistic (OS) CFAR can provide performance which is nearly equal to the cell average method when in a noise only environment, and can provide improved performance when operating in an environment containing clutter and/or multiple targets.
An OS CFAR Detector performs the following functions:
1. Inputs all video samples within N/2 samples on either side of the sample cell which is being examined for the presence or absence of a Target. "N" is referred to as the size of the "window" examined by the circuit.
2. Examines the amplitude of the N samples and assigns each of them a rank according to relative amplitude. This is analogous to sorting N different numbers in order by the values of the numbers.
3. Selects the sample which has a Predetermined Rank (RT) from the sample set. The amplitude of this sample is used as a basis for setting the Target Detection Threshold.
4. Determines the number of samples (Nc) which exceeds the average noise level by a predetermined amount. When this number exceeds certain predetermined values the circuit will adjust RT to prevent an increase of the false alarm rate in severe clutter. As RT changes the circuit automatically adjusts the detection Threshold offset (TD) to maintain the desired probability of false alarm.
All of the above functions must be performed for each video sample point and in a parallel fashion such that the detection function can take place in real time at a rate that matches the video bandwidth.
As may be appreciated, an essential feature of an OS CFAR detector is the ability to rank order a set of numbers. Prior methods used for rank ordering a set of numbers involve algorithms implemented on programmable computers. These algorithms require multiple passes through the sample set. Each pass requires moving data among locations in memory and comparing operations in an arithmetic logic unit (ALU). The disadvantage of these prior techniques is that they are extremely slow.
It may further be seen that in the OS CFAR application, the sample cell or test cell is excluded from the ranking process. U.S. Pat. No. 4,649,394 to Minker and Rohling discloses an OS CFAR radar in which rank ordering is proposed, but does not disclose circuitry capable of rank ordering the number set while excluding the sample or test cell from the ranking process. Such capability is also lacking in other number ranking circuits, for example, as disclosed in U.S. Pat. No. 3,927,391. The circuitry of that patent further does not output or select a particular value, but rather outputs only the rank of each member in a set of numbers. Also, according to the '391 patent, if two samples have the same value, they are given identical rank, which precludes the circuit from outputting a single member of a particular rank.
A rank ordering logic circuit which updates rank order within a single clock cycle is disclosed in U.S. patent application Ser. No. 220,138 by Ronald L. Engelsman, assigned to Hughes Aircraft Company. While providing an improvement in the art, the Engelsman circuit is limited to a FIFO (first-in/first-out) scheme of window management and a fixed window size while a data stream is being processed. In some radar applications, it proves desirable to have the capability to change the window size from beam to beam. Such flexibility is unavailable in the Engelsman design because all cells are not identical. Additionally, the target cell location is fixed in the Engelsman circuit, whereas it may prove desirable to have the capability to vary the target cell location.